You know how in the newer games, you can walk behind things (like rooftops, treetops, etc.)? Well, today I will explain how you can do this in Gold and Silver. Now, please note, there are a few limitations in this, but it is incredibly simple. With some creativity, it can be quite handy. If someone used a bit of ingenuity to modify an ASM routine, it could possibly be even more useful.

What you will need:

TLP
Tileset editor or Hex Editor
Creativity

This couldn't be simpler. To begin, open your rom in TLP and go to the offset x14639

Here you will see an odd tile, a white tile with a few black specs on it, and a grass tile, like this:

We will begin by erasing the grass tile (and optionally the odd tile). This will remove the green bushy overlay from your sprite when walking in tall grass, but this isn't a big deal. (For an example, play Pokémon Christmas and notice the way tall grass behaves)

Now what this does is give us the opportunity to have some fun. On any tile you want the player to be able to walk behind, set its collision byte to x10. This collision byte calls the bushy grass effect, without loading wild Pokémon encounters.

Now we just get creative. When Hiro walks onto a tile with this collision effect, the bottom 2 tiles and the bottom half of the upper two tiles will be loaded ABOVE Hiro, with the first colour in the pallet loaded as transparency. To explain better, here is a picture.

In this image, the Black pixels are the area that will load behind Hiro, and the Red part will be displayed ABOVE Hiro, with the first colour loaded as transparent. While this makes roof walking difficult (due to the half tile nature currently employed) we can still use it for some fun things. Like this:

If you look, the tree top is showing up on top of Hiro. This would be a lot better if someone figures out how to hack the routine so that all 4 tiles load above the player instead of the way it is now, but with creativity you can still make this way work, and do some fun things with it.

Unfortunately, this only works well for things that don't need to fully overlap the player. So you can walk behind treetops, roof edges (if you get creative) and things like that, but making archways to walk completely under would render them strangely. The player's head would stick through the top the way it does when walking in grass. So it isn't perfect by any means, but it can still add some cool effects to a Gen II hack. All you really need is creativity.